So My 91 RS began to overheat. Naturally i replaced the thermostat first (185). I noticed the fan was jogging on the motor shaft so i replaced it with a universal fan (hayden brand). I did a radiator flush and it is STILL overheating (260+). There is no water coming from the weephole of the water pump, this is the only part of the cooling system i have not changed or flushed out. I do live in Phoenix where its 110+ for 5 months of the year which is when this issue popped up. is it just running hot due to outside temp. Or is there something I'm missing or could try? Anything will help at this point.

 

Have you pressure tested the system? Is it just when sitting still or at highway speed also?

If the system will hold pressure and its overheating on the highway I would invest in a heat scanner and verify the entire radiator is getting hot. Partially plug radiator is a lot more likely then a none pumping pump but both possible. Blown head gasket is another possibility.

 

I don't have a pressure tester or means to get one. It isn't running through water so I'm pretty sure it's not the heads. I had the radiator out to put the new fan in and its clean. The one time I drove it on the freeway it didn't get hot, but it was at night and I had the heater on to draw heat out. I've read something about the springs in the lower radiator hose could be collapsing not allowing for full circulation. I haven't checked yet but that is my next step.

 

I have not seen a hose with a spring in it for 20 years,,,, When your system over heating you should have 15 lbs of pressure in there. Nothing is collapsing. It take very high RPMs to collapse a hose. This is an issue on circle track cars that run 7000 RPMs for 10 minutes straight.

 

Ok the hose isn't collapsing.I also checked the radiator levels and in only took a few ounces before it was full again so im pretty positive it isn't heads. my next step is getting an infrared thermometer and making sure the cars gauge isn't out of whack. after that I'm lost.

 

Coolant lose is a sign of an advanced blown head gasket. Mechanics use a gas analyzer to test for a blown head gasket by check for hydro carbons. We would not need that is all you had to do is check the fluid level. If you are burning coolant you are already to the point you would have staining on the tail pipe and at least one very clean spark plug.

In the early stages of a blown head gasket the cylinder pressure (1200-1500 PSI) will push into the coolant system and cause gas pockets that does not let the coolant flow well and depending where the high pressure gas is forming a pocket will detriment if or how bad it will overheat. Its later in the head gaskets failing process that it sucks coolant in on the intake stoke. I am not saying that is what is wrong. I only saying it is one of the possibilities. This would only apply to a head gasket that failed over time not one that went in a single over heat event. FYI, most chain parts stores will loan you a pressure tester.

I am just telling you the diagnosing procedure for the coolant system. If the system can not hold pressure the coolant will boil and create the same exact symptoms as a blown head gaskets, It will also help to cause a head gasket failure.

Once you are sure the system will hold pressure you can feel around on the radiator for cold spots, back in the day we did this by hand. You want to do it as the car warms up the first time. If some areas get hotter faster the cooler areas are being blocked by something inside the radiator. "normally this is the bottom of the radiator and just crude" In most cases this can be flushed but in some cases even that is not enough and the radiator is scraped.

This is an easier way to do it to check temps.
https://www.harborfreight.com/non-co...ing-61894.html

Next I would verify the timing, but on your engine that is not adjustable, verify its not running lean (it should set the Check engine light). Then replace the water pump. Note replacing the water pump is the last thing on the list because its very hard to test and it VERY rare the a water pumps stop pumping, basically the impellors have to rust away.

A couple of things to think about.

Defective thermostat, I have had this twice. In class they suggest testing the thermostat on troubled cars and only installing proven thermostat while trouble shooting an over heating issue. Just boil some water and dunk it.

If the car does not overheat at 50 miles per hour on a hot day you could just have low CFM going across through radiator, just about every time I have seen the it is because of a shroud being removed, fans weak and even a couple of fans turning the wrong way can do it also.